1. Field of the Invention
The present invention relates to an optical time domain reflectometer which is especially suitable for optical communication.
2. Background Art
Up until now, OTDR (Optical Time Domain Reflectometry) has been developed for detecting breaking points or loss of connection or the like in laid optical cables. By this OTDR, the position of a breaking point or loss of connection and the like is detected in a manner such that a high power optical pulse is incident on an input end of the laid optical cable, and a crest value and arrival time of the response light, which has been returned to the input end by back-scattering or reflection, are measured.
FIG. 2 shows an example of the optical time domain reflectometer to conduct the detection by the OTDR described above. In the figure, reference numeral 1 indicates a high power light source generating a light pulse, the repeating frequency of light pulses generated by light source 1 being set in accordance with the accuracy of the measurement distance. Reference numeral 2 indicates a light branch device which makes the light pulse, which has been inputted from the high power light source 1 to an input end 2a, to be incident on an end portion 3a of an optical cable 3 to be measured. The light branch device also outputs response light reflected from the cable 3 to an output end 2b. In addition, cable 3 to be measured is a bundle of optical fibers, each of which has been elongated by connecting a number of optical fibers together. Reference numeral 4 indicates a light receptor which converts the response light from the output end 2b of the light branch device 2 to an electric signal and supplies the electric signal to an amplifier 5.
In this arrangement, when high power light source 1 generates a high power light pulse, the light pulse is incident on the end portion 3a of cable 3 to be measured via light branch device 2. Response light which has been returned to the end portion 3a by back-scattering or reflection in the cable 3 is supplied to light receptor 4 via the light branch device. In light receptor 4, the response light is converted into a corresponding electric signal which is subsequently amplified by amplifier 5 and supplied to measurement circuit 20. The measurement circuit 20 conducts the detection as described above in accordance with the supplied electric signal.
In addition, the above-mentioned optical time domain reflectometer is also used for optical communication in which a very long optical cable is used. Therefore, it is necessary to enlarge a possible range of the level of the response light to be measured (referred to as a "dynamic range", hereinbelow). Until now, developmental efforts for increasing the power of the light source or sensitivity of the light receptor, or decreasing the loss related to the insertion of each optical circuit and the like have been made for enlarging the dynamic range. In recent years, it has been believed that a ring laser apparatus 6, whose approximate arrangement is shown in FIG. 3, may be used for a higher power light source.
In the ring laser apparatus 6 of FIG. 3, reference numeral 7 indicates a excitation light source which generates continuous light having a specified wavelength (e. g., 1.48 .mu.m). Reference numeral 8 indicates an optical synthesizer which synthesizes the continuous light inputted from the excitation light source 7 to the input end 8b and the continuous light supplied to the input end 8a via optical switch 11 which receives the continuous light and generates a light pulse by changing its state.
Optical fiber 9 is doped with, for example, one of the rare earth elements such as erbium (Er); thus, erbium ions (Er.sup.3+) are distributed in the cable. When the continuous light is incident from excitation light source 7 on the input end of the optical fiber 9 via optical synthesizer 8, the optical fiber 9 is excited. Reference numeral 10 is a light branch device which directs the supplied optical pulse to switch 11 or its output end for an optical cable to be measured.
By the apparatus described above, when continuous light is generated from the excitation light source 7, optical fiber 9 is excited. At this time, switch 11 is set "on", and a closed loop is formed; thus, the continuous light is amplified. Then, if switch 11 conducts an on/of operation, a light pulse is generated, and after that, if switch 11 is set "on", the light pulse is amplified in the closed loop. Therefore, the crest value of the light pulse outputted from the output end of the light branch device 10 is very large. In addition, details of the ring laser apparatus 6 are disclosed in the specification of Japanese Patent Application, Laid Open First Publication No. Hei 05-21880 which the present applicant formerly filed.
However, if ring laser apparatus 6 is used in the above-mentioned conventional optical time domain reflectometer, it is necessary to use two light branch devices 2 and 10; this raises the system cost. In addition, these light branch devices 2 and 10 are inserted in the path of the light pulse, and the loss of the insertion is thereby large. Accordingly, there has been a problem that the dynamic range of the OTDR cannot be enlarged to the degree anticipated.